Sawing sapphire (alpha alumina, α-Al2O3) into wafers is a challenge as the material has a hardness of 9 on Mohs scale (diamond having the highest hardness of 10 on Mohs scale). Currently the technology of using diamond loaded dicing blades is being overtaken by the use of multiwire saws for reasons of economy. In a multiwire saw, a single sawing wire is led over grooved capstans thereby forming a web of closely spaced apart wire lengths. By reciprocating the wire in a back and forth movement and pushing the sapphire boule into the web under supply of a coolant, the sapphire boule gets cut into wafers. Instrumental in the whole process is of course the sawing wire that is almost exclusively of the fixed abrasive sawing wire type.
In a ‘fixed abrasive sawing wire’ tiny cutting diamonds are firmly attached to the surface of a fine wire of high strength. The finer the wire, the lower the kerf loss will be which is an advantage as then in one cutting cycle more wafers can be obtained from the same boule in an equal time span. Still the cutting speed is low: about 3 to 4 hours is needed to saw a 2 inch (50.8 mm) boule but this is outweighed by the parallel cutting of many wafers in one cycle. In addition the wire wears in the process and on average about three to eight meter of wire is needed to cut a single wafer. As only diamond abrasive particles are used, the wire tends to be expensive.
Currently fixed abrasive sawing wires are made according a number of processes resulting in strongly differing products:                A. The abrasive particles can be held in a resin bonded to the substrate wire. As the resin tends to wear rather quickly thereby losing the abrasive particles, the wear of the wire is rather high.        B. EP 2390055 describes a fixed abrasive sawing wire wherein abrasive particles are fixed in a metal layer made of a low melting point solder based on zinc or tin. The abrasive grains are made to adhere well to the solder by mediation of a high melting point metal that is present around the abrasive particle. The disclosure particularly mentions that crushed diamonds are preferred as abrasive particles as these tend to have a larger contact surface with the solder.        C. EP 2277660 describes a fixed abrasive sawing wire wherein abrasive particles—diamond and cubic boron nitride are preferred—are encapsuled in a nickel phosphorous coating that is at least partly crystallized. The encapsulated abrasive particles are electrochemically coated on a magnetic linear body such as a steel wire. No information is given on the morphology of the abrasive particles.        D. EP 2464485 discloses an abrasive article including abrasive particles bonded to an elongated body that is used for the cutting of sapphire. Abrasive particles are pressed into a metallic bonding layer further possibly covered by a coating layer. Specific about this wire is that the size distribution of the abrasive grains is non-gaussian and substantially uniform while extending over a wide yet precisely limited range of from about 1 micron to about 100 micron.        
In commercial fixed abrasive sawing wire exclusively ‘crushed diamonds’ are used. These are obtained by fragmenting larger man-made or natural diamonds by crushing (ball mills, shock impacting or other techniques). Man-made diamonds are preferred for their predictable and reproducible properties. They are made from a mixture of graphite and metallic catalysts under extreme pressure and temperature and the processing and starting products allow to tune the resulting diamonds in size, strength, friability and shape. Hence, more friable diamonds are used for crushing. The resulting mix is classified usually by means of sieving but for smaller grit sizes other techniques must be used (air blow separation, sedimentation, elutriation, or other techniques). In this way fine powders can be obtained in an economical way.
In a sawing wire for cutting silicon wafers—for example for solar cells or for the semi-conductor industry—a more friable diamond is preferred as during use new cutting edges then appear i.e. the wire is self-sharpening. Moreover, the irregular shape of the diamond results in a better retention in the matrix.
An extreme embodiment of irregular shaping is demonstrated in WO 2011/014884 A1 wherein a fixed abrasive sawing wire is disclosed having surface modified diamond particles. The surface of the diamonds has been treated in order to obtain a very rough outer surface and is largely aspheric. Although for certain applications it is believed that such surface will result in better retention and self-sharpening, this will not be the case for very hard materials like sapphire.
Indeed, the cutting of extreme hard materials such as sapphire poses other challenges in that the friable diamonds will wear too fast which must be compensated by having more diamond on the surface of the wire and or a faster use of the wire during cutting. Also—due to the hardness of the material—a higher pressure must be exerted in the cut to push the diamonds with sufficient force against the sapphire. This can be achieved by applying higher tensile forces on the wires in the web. Hence, this implies also that thicker wires must be used leading to an associated increased kerf loss.